>if you learned them once you can learn them again
I doubt this personally. I didn't have any problems with the partial differential I was set. I mean on a general level that something once learnt can be relearned.
You may not be as sharp at it as you were when you spent hours pounding it into your head on a frequent basis, but it's surely within a person's grasp. At least as far as math goes; something like language fluency may be another story.